The present invention relates to a storage capacitor of a planar display and a process for fabricating the storage capacitor, and more particularly to a storage capacitor applied to a top gate thin film transistor liquid crystal display (TFT-LCD) and a process for fabricating the same.
Nowadays, LCD has been applied to many fields of applications such as cellular phones, digital still cameras (DSCs), digital video cams (DVCs), camcorders, global positioning systems (GPSs), personal digital assistances (PDAs), personal computers (PCs), notebooks, and televisions (TVs). LCD has been playing an important role and acting as developing trend in the future with far more characteristics and advantages like thinness, low energy consumption, high resolution, high lightness, etc. Among many kinds of LCDs, TFT-LCD is so far the most popular planar display. Please refer to FIG. 1 which is a circuit diagram showing one pixel cell of a TFT-LCD. This pixel cell mainly consists of a switching unit (TFT) 11, a LCD unit 12, and a storage capacitor 13. The storage capacitor 13 is electrically connected to the LCD unit 12 in parallel to compensate the charge storage capacity of the LCD unit 12 which is also of a capacitor structure. Otherwise, the voltage of the LCD unit 12 may fall undesirably significantly due to voltage leakage after turning off the switching unit 11.
A low temperature polysilicon (LTPS) process is developed for manufacturing the TFT array of the LCD. The low temperature polysilicon has the characteristic of faster electron mobility than the amorphous silicon (a-Si) over 100 times. That explains why each pixel of low temperature polysilicon has faster response time and smaller outlined dimension comparing with amorphous silicon. FIG. 2 shows the conventional process for manufacturing a top gate LTPS-TFT on a glass substrate via a complementary metal-oxide-semiconductor (CMOS) manufacturing process. Please refer to FIG. 2(a). A buffer layer 21 (e.g. a silicon dioxide layer) and an intrinsic amorphous silicon (i-a-Si) layer are formed on the glass substrate 20. The intrinsic amorphous silicon layer is transformed into an intrinsic polysilicon (i-p-Si) layer 22 by a laser crystallization process. After the micro-photolithography and etch steps, the first intrinsic polysilicon structure 221, second intrinsic polysilicon structure 222, and third intrinsic polysilicon structure 223 are formed, as illustrated in FIG. 2(b), and later provided as parts of n-channel TFT, p-channel TFT, and storage capacitor, respectively.
A photoresist layer is then deposited on the resulting strcuture of FIG. 2(b). Afterwards, a micro-photolithography process is performed to form masks 23 on the first polysilicon structure 221 and second polysilicon structure 222. Further referring to FIG. 2(c), an n+-doping step is performed on the polysilicon layer with the mask of the photoresist 23 to define the source/drain regions 2211 of the n-channel TFT and the lower electrode 2231 of the storage capacitor. After removing the photoresist masks 23, an insulating layer (e.g. silicon dioxide layer) is applied to the resulting structure to form a gate insulator layer 24 as shown in FIG. 2(d).
Please refer to FIG. 2(e). After sputtering a gate metal layer on the gate insulator layer 24, the gate metal layer is patterned and etched to form a gate metal structure 251 and the upper electrode 252 of the storage capacitor. Then, the gate metal structure 251 serves as a mask for a following lightly nxe2x88x92-doping step to form a lightly doped drain structure 2212. Afterwards, another photoresist layer is applied to the resulting structure and patterned and etched to form masks 26 on the n-channel TFT region and the storage capacitor region. A p+-doping step is then performed to form the source/drain electrodes 2221 of the p-channel TFT region, as shown in FIG. 2(f).
After removing the photoresist masks 26, an inter-layer dielectric layer 27 is provided, and a plurality of contact holes 28 are created by penetrating through the dielectric layer 27 at appropriate positions, as shown in FIG. 2(g). Then, a metal layer is deposited by sputtering to fill the contact holes 28, and patterned and etched to form gate contact structures 290, source/drain contact strictures 291, and electrode contact structures 292 of the storage capacitor. In the above structure, the lower electrode 2231 of the storage capacitor is made of highly doped n+-polysilicon and has excellent conductivity. Therefore, the capacitance provided by the lower electrode 2231 and upper electrode 252 is enough for most situations.
For some cases where the presence of the LDD stricture is not critical, the above process will be cost-inefficient. Therefore, other processes omitting the relating steps of the LDD structure are developed to reduce the production cost. One of the examples is illustrated with reference to FIGS. 3(a) and 3(b). After the steps similar to the ones illustrated with reference to FIGS. 2(a) and 2(b) are performed, a gate insulator 24 (e.g. silicon dioxide layer) and a gate metal layer are applied to the resulting structure. A photolithography process is executed to pattern the gate metal layer and thus form the gate structures 35, 36 and upper electrode 37 on the intrinsic polysilicon structure 221, 222, and 223, respectively. The gate structures 35 and 36 have smaller size than the first and second intrinsic polysilicon structures 221 and 222, and the upper electrode 37 is smaller than the lower electrode 223. Afterwards, a photoresist layer is deposited and patterned to form a mask 38 on the second intrinsic polysilicon structure 222. Then, an n+-doping procedure is performed on the portion exposed from the mask 38 to define the source/drain electrodes of the n-channel TFT. The following procedures are similar to those shown in FIGS. 2(f) to 2(h) and will not be described again herein.
In this process, the upper electrode 37 is formed prior to the n+-doping procedure. Due to the shield of the upper electrode 37, as shown in FIG. 3(b) or 3(c), the lower electrode 223 cannot be well defined by the n+-doping procedure. Therefore, the conductivity of the storage capacitor is not satisfactory.
Occasionally, essentially one of the n-channel TFT or the p-channel TFT is required and thus formed in the TFT matrix. FIGS. 4(a) and 4(b) show an exmaple that only the p-channel TFT is formed. The steps and numeral references are similar to those shown in FIGS. 3(a) and 3(b) except that the steps specific to the n-channel TFT region are omitted. Along with the p+-doping step for forming the source/drain electrodes of the p-channel TFT, the polysilicon structure 223 is doped to form the lower electrode of the storage capacitor. The following procedures are similar to those shown in FIGS. 2(g) and 2(h) and will not be described again herein. Due to the shield of the upper electrode 37, the lower electrode cannot be well defined by the p+-doping procedure. Therefore, the conductivity of the storage capacitor is not satisfactory.
An object of the present invention is to provide a storage capacitor structure having improved capacitance.
Another object of the present invention is to provide a process for fabricating a storage capacitor having improved capacitance.
A first aspect of the present invention relates to a storage capacitor of a planar display. The storage capacitor includes structures of a substrate; a lower electrode formed on the substrate and made of a semiconductor material; an insulator layer formed on the lower electrode; and an upper electrode formed on the insulator layer and having a dopant passage therein for allowing dopants to penetrate therethrough to reach the lower electrode. The storage capacitor is especially suitable for a top gate thin film transistor liquid crystal display (TFT-LCD), preferably a low temperature polysilicon (LTPS) TFT-LCD. Generally, the lower electrode is electrically connected to the drain electrode of a top gate TFT of the LCD, and the substrate is generally a transparent substrate.
In an embodiment, the lower electrode is made of doped polysilicon, the insulator layer is made of silicon dioxide, and the upper electrode is made of metal.
Preferably, the upper electrode is in a shape selected from a group consisting of a comb, a fishbone and a net so as to provide the dopant passage therein.
A second aspect of the present invention relates to a process for fabricating a storage capacitor of a planar display. The process includes steps of providing a substrate; applying a semiconductor layer onto the substrate; defining a lower electrode region in the semiconductor layer; applying an insulator layer onto the lower electrode region; forming an upper electrode having a dopant passage therein on the insulator layer; and doping the lower electrode region through the dopant passage to form the lower electrode.
Preferably, the lower electrode region is defined by a micro-photolithography procedure perfomed on the semiconductor layer, and the upper electrode is formed with the dopant passage by a micro-photolithography procedure perfomed on a metal layer.
Preferably, the doping step is perfomed along with the source/drain formation of a TFT of the planar display.
A third aspect of the present invention relates to a process for fabricating a storage capacitor of a planar display comprising steps of: providing a substrate; applying a first semiconductor layer onto the substrate; defining a lower electrode in the first semiconductor layer; applying an insulator layer onto the lower electrode region; applying a second semiconductor layer onto the insulator layer; defining an upper electrode in the second semiconductor layer; performing a first doping operation with a first doping intensity to provide dopants in the lower electrode; and performing a second doping operation with a second doping intensity less than the first doping intensity to provide dopants in the upper electrode.
Preferably, the first and second semiconductor layers are both made of polysilicon.
The first doping operation can be performed before or after the second doping operation. Preferably, the first and the second doping operations are performed successively.
A fourth aspect of the present invention relates to a storage capacitor of a planar display, comprising: a substrate; a lower electrode formed on the substrate and made of a first doped extrinsic semiconductor material; an insulator layer formed on the lower electrode; and an upper electrode formed on the insulator layer and made of a second doped extrinsic semiconductor material.
Preferably, the first and second doped extrinsic semiconductor materials are both polysilicon.
Preferably, the lower electrode is electrically connected to the drain electrode of a top gate TFT of the LCD.